Baking multiple normal maps into one
Facing the same problem now, really confusing, but I've just found a solution.

Normal maps contain non color data, and when baking normal map images (e.g. by shader emission, when noncolor is enabled) doesn't gives you what you expect.

You should enable color for each normal map images for baking as a diffuse or emission (of course it changes preview result). 

Note that it will not yield exact results, but very close to original, maybe because I'm not using EXR, but I'm fine with the result.

English is not my first language so I hope its understandable.